What is a function parameter =[] for?

[Oscar Benjamin]

On 24 November 2015 at 15:27, Ned Batchelder <ned at nedbatchelder.com> wrote:
> On Tuesday, November 24, 2015 at 10:10:51 AM UTC-5, Antoon Pardon wrote:
>> Op 24-11-15 om 15:18 schreef Ned Batchelder:
>>
>> > 2) In Python, "value" means, what object does a name refer to, or what
>> > object did an evaluation produce.
>>
>> I don't think this is correct because that would imply that objects don't
>> change values (since the value would be the object).
>
> Sorry, I should have been more precise.  Python documentation suffers from
> this two-meaning world also.  There are places where "value" is used in
> this second sense, perhaps more places than you realize.  I know when I
> wrote "Python Names and Values", I meant it in this second sense.

Hi Ned, I read your talk on this subject which was linked to further
up in the thread and it is excellent so thanks for that. I forwarded
it to a colleague for inspiration in our own introductory programming
(with Python) teaching unit.

However when I read it and imagined explaining it to our students I
found myself wanting to adjust the terminology. I would not use the
word value in the way that you suggest. Rather I would want to
contrast "names" (or "references"), "objects" and "values". I think
that many things can be expressed more cleanly with these terms and I
think it corresponds more to the way in which I see Python programmers
use these terms.

So I would say that an object has a value. Immutable objects have
unchanging values but the value of mutable objects can change. The
expression "a is b" determines whether or not a and b are bound to the
same object whereas "a == b" determines if the objects bound to a and
b have the same value. To copy an object is to create a new object
having the same value as the copied object. To mutate an object is to
change its value etc.

Earlier in this thread you said:
"""
In Python, a default value expression
for a function argument is evaluated only once, when the function is defined.
That value (in this case, the actual list) is stored with the function. The
expression is not stored, the value of the expression is. That value (the
actual list) is supplied as the value of the argument if no other value is
supplied.  If you modify that value in the function, the value is modified,
and used again at the next function call.
"""
I think that's a good example of where it would be clearer to
distinguish between objects and values, rather than using the word
value for both. In particular what is stored with the function is the
*object* that results from the default value expression. The *value*
of that object may change if it is mutated but it will still be the
same object each time the function is called (without a corresponding
argument being passed).

--
Oscar